Magnetic Resonance Imaging (MRI) technology is commonly used today in larger medical institutions worldwide, and has led to significant and unique benefits in the practice of medicine. While MRI has been developed as a well-established diagnostic tool for imaging structure and anatomy, it has also been developed for imaging functional activities and other biophysical and biochemical characteristics or processes (e.g., blood flow, metabolites/metabolism, diffusion), some of these magnetic resonance (MR) imaging techniques being known as functional MRI, spectroscopic MRI or Magnetic Resonance Spectroscopic Imaging (MRSI), diffusion weighted imaging (DWI), and diffusion tensor imaging (DTI). These magnetic resonance imaging techniques have broad clinical and research applications in addition to their medical diagnostic value for identifying and assessing pathology and determining the state of health of the tissue examined.
During a typical MRI examination, a patient's body (or a sample object) is placed within the examination region and is supported by a patient support in an MRI scanner where a substantially constant and uniform primary (main) magnetic field is provided by a primary (main) magnet. The magnetic field aligns the nuclear magnetization of precessing atoms such as hydrogen (protons) in the body. A gradient coil assembly within the magnet creates a small variation of the magnetic field in a given location, thus providing resonance frequency encoding in the imaging region. A radio frequency (RF) coil is selectively driven under computer control according to a pulse sequence to generate in the patient a temporary oscillating transverse magnetization signal that is detected by the RF coil and that, by computer processing, may be mapped to spatially localized regions of the patient, thus providing an image of the region-of-interest under examination.
In a common MRI configuration, the static main magnetic field is typically produced by a solenoid magnet apparatus, and a patient platform is disposed in the cylindrical space bounded by the solenoid windings (i.e. the main magnet bore). The windings of the main field are typically implemented as a low temperature superconductor (LTS) material, and are super-cooled with liquid helium in order to reduce resistance, and, therefore, to minimize the amount of heat generated and the amount of power necessary to create and maintain the main field. The majority of existing LTS superconducting MRI magnets are made of a niobium-titanium (NbTi) and/or Nb3Sn material which is cooled with a cryostat to a temperature of 4.2 K.
As is known to those skilled in the art, the RF coils generally are configured to selectively provide excitation (transmit coil) and reception (receive coil) of an MRI signal. Three kinds of RF coils may be employed in common MRI scanners: a transmit coil, a receive coil, and a transceiver coil. The transmit coil is usually disposed close to the magnet so it can provide homogenous RF excitation over the whole FOV (field of view). Although the transmit coil may also be used for reception, the transmit coil is not necessarily a good receive coil for detecting the MRI signal because it is usually too far away from the patient. Accordingly, typically a separate receive coil is used, which is usually small in size and closely wrapped around the body part of the patient to be imaged so that the signal or signal-to-noise ratio (SNR) is higher. However, different receive coils have to be used for different body parts. There are usually 6 to 10 different receive coils for each MRI scanner to fit different body parts and different sizes of them. It takes time to change a coil. Rather than using separate transmit and receive coils, a transceiver coil, which combines the functions of the transmit coil and receive coil, may be implemented for some MRI scanning applications. It is common for the transmit coil of some MRI systems to be implemented as a transceiver coil. This kind of coil can image a large FOV so patient positioning is easier than using a small receive coil. However, the SNR of the images acquired by a transceiver coil is low, thus precluding diagnostic imaging when using such coils.
One way to increase the SNR of an image is to use an RF coil array. As the size of the receive coil decreases, the coil noise increases, which will affect SNR of the image. Such arrays generally have been implemented as surface coil arrays that are disposed on or near a specific body part to be imaged (e.g., head, orthopedic, breast imaging).